2

ReferencE
Models of Network
These days technology is increasing at very fast pace. An organization takes benefit
of this highly growing technology by integrating various components supplied by differer
vendors. The components supplied by diferent vendors may not have compatibin,
and may be diffcut to operate simultaneously. The mismatches between the component
lead to the development of integrated approach, known as reference models.
The reference model describes various Components of network, how they interac
(ineraces) and the set of rules (known as protocols). The reference model describes
the architecture of the network. It identifies the mapping between the different layes
of network architecture of the network.

In this chapter, we will discuss one important network architectures, namely, the
OSIreference model.

2.1 The OSI Reference Model

This layered network architecture model was developed by International Organization
for standardization (|1SO) located in Geneva, Switzerland, and consutative cammite
forinternational Telephone and Telegraph (CCITT) t (OSI) was designed to develop
common standards of network architecture throughout the word. It provides a generd
concept of inter proces_ communication in such a way that any open system may
communication with another open system technically without any problems. lt 1s a
seven layered architecture model. Fig. 2.1 shows the seven layered Architecture o
OSl models along with protocols and interfaces.
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 /The model is designed in a highly structured way, It defines a separate set of
for each layer and hence each layer has a
protocos
specific independent function.
The OSl model does not perform any function in the
communication process. Ine
actual work is done by the appropriate software and hardware. The OSl model
simply defines which tasks need to be done and which protocols will
handle those
tasks. In actual implementation of the seven layers, the first three
layers are likely
to be in hardware, the next two layers in the operating
system, the presentaton
layer in library subroutines in the user's address space, and the
application layer in
the user's program,

Computer A Name of Unit

Service Intertace Computer B exchanged

Layer 7
Application, Application
Layer Protocol APDU
Layer

Layer 6 Presentation Presentation

Layer Protocol PDU
Layer

Layer 5 Session Session

Layer Protocol * SPDU
Layer

Layer 4 Transport Layer *.**.Protocol . Transport Layer TPDU

Layer 3 Network Layer eesuna

Protocol Network Layer Packet

Layer 2 Data Link Layer Protocol .. Data Link Layer Frame

Layer 1 Physical Layer. . Protocol Physical Layer bit

Transmission Media Transmission Media

FIGURE 2.1

Let us discuss the seven layers in brief.

2.1.1 Physical Layer

This is the lowest layer in the model. This layer is responsible for activating,
maintaining and deactivating a physical circuit between two end systems.
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 The Physical Layer is responsible for sending bits (bits means binary
and 0's) from one computer to another. Thatt is, it may convert the digits, i.e. t
into electric signals, light signals, or electromagnetic sequenceon
signals, depending
of bit

the two nodes are on a cable circuit, fiber-optic circuit.,-or

respectively. Even electrical details, such as how many volts to use for O
microwavelradio whetcircuihter
and
how,many bits can be sent per second, and whether transmission
can
take place
onlyin one direction or in both directions simultaneously, are decided by
the
phyical
layer protocols. In addition,the physical layer protocols also deal withtthe
details, such as the size and shape of the connecting plugs, the numbermechani
of
cal
the plugs, and the function of each pin.
pins in
Passive hubs, simple active hub, terminators, couplers, cables, connectore
repeater, multiplexers, transmitter and receives are devices associated wi%
physical layer. The position of the physical layer with respect to the transmission
medium and the data link layer is shownin figure 2.2

From data link layer To data link layer

Layer 2 data Layer 2 data

Physicai Physical
Layer 10101000110010] 10101000110010 Layer

Transmission Medium

FIGURE 2.2

Functions of the physical layer are summarised as :

Physical layer is responsible for sending bits from source computer t
destination computer.
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 2. Bit transmisslon encoding
It defines the bit transmission encoding i.e. how 0's and 1's are changed to
signals.
3. Transmission rate
lIt defines the transmission rate (or data rate) i.e. the number of bits transmitted
per second.

4. Line configuration
(a) Point-to-point
(b) Multipoint
So, it deals with line configuration.
5. Transmission mode
(a) Simplex
(b) Half duplex
(c) Full duplex
It defines the transmission mode between tWo devices.
6.
Topologyi
It deals with physical topologies i.e. star, ring, bus, hyprid or mesh etc.
7. Multiplexing
It deals with combining several data channels, into orie.
8. Media bandwith (Data transmission speed)
(a) Narrowband
(b) Voiceband
(c) Broadband
9. It deals with the synchronization of the transmitter and receiver.

2.1.2 Data Link Layer

The main task of the data link layer is to provide error free transmission. The physical
layer simpy transmits the data from the sender's node to th¹ receiver's node as
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raw bits. It is the responsibility of the data-link layerto detect. and correct
in the transmitted data. any erra
It accepts packets from the network layer and splits the packets into
are transmitted by sender through physical layer as shown in fig. 2.3 trames whic,
From network layer From network layer
Layer 3 data Layer 3 data

Data Frame Frame T, H. Data

Link
T H, Link
Layer Layer

10101000110010 10101000110010

To PhysicalLayer ToPhysical Layer

Transmission Medium

FIGURE 2.3

It also provides flow control and prevents overburdening of receiver with too much
data at any instant. A Cyclic Redundancy Check (CRC) added to the data frame
can detect damaged frames and the data link layer in the receiving computer ca
request that the information be present. The data link laver can also detect when
frames are lost and request that those frames be sent again.
Bridges, intelligent hubs and network interface cards are devices typicaly
associated with data link layer.
The data link layer splits into two sublayers.
Logical Link Control (LLC), which controls establishes and maintains line
between the communicating devices.
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 Media Access Control(MAC), which controls the waymultiple devices share
the same media channel.

Functions of data link layer are summarised as :

Frammings
It divides packets received from the network layer into manageable data units
called frames.
2 Physical addressing
Itadds a header to the frame to define the physical address of the sender and/
receiver of the frame.
Error control
Error control is main task of the data link layer. It uses the protocols to detect
errors inframes and to ensure transmission of correct frames.
4. Flow control

It provides a flow control mechanism to avoid a fast transmitter from over

running a slow receiver by buffering the extra bits.
5. Access control
The data link layer protocol determines which device has control over the link
at any given time, when two orore devices are connected to the same link.

2.1.3 Network Layer

The network layer establishes alogical connectiQn between sender and user by
providing a logical path between them. This layer switches and routes message
páckets as necessary to get them to their destination The layer is responsible for
addressing and delivering message packets. .Network layer makes routing
decisions and forwards the packets for devices that are farther away than a
single link.
The network layer also may break large packets into small chunks if the packet is
larger than the largest data frame the data link layer accept. The network reassembles
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the chuks into packets at receiving end. Routers and gateways
operate
networklayer. The relationship of the network layer to the data link and In th
layer is shown in Fig. 2.4
tra nspon
From transport layer To transport layer
Layer 4 data Layer 4 data

Network
Layer Network
Layer

H, Packet Packet H,

Layer 3 data Layer 3 data

To data link layer To data link layer

FIGURE 2.4

It receives services from the data link layer and provides services to the
transpot layer.
The function of network layer are :
Internetworking
This is the main duty of network layer. It provides the logical connection
between different types of networks.
2 Addressing
Addressing is necessary to identify each device on the Internet uniquely. This
is similar to a telephone system.
3. Routing
In a network, there are multiple routes available from a source to a destinatio
and one of them is to be chosen. Routing algorithms are designed to find ie
most efficient paths between the source and destination nodes of a messagu
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 Packetizing
It breaks the larger packets into smaller packets if the packet is larger than
the largest data frame the data link layer accept.This is called as packetizing
5. Data Switching
| The process of transferring packet (data blocks) from source to destination
node is called data switching Hence, network layer is concerned with the
circuit, message or packet switching.
6. Congestion control
Congestion in a network may occur when the load on the network i.e. the
number of packets sent to the network is greater than the capacity of the
network. Network layer controls the congestion.
7.
It provides connection services, including network layer flow control, network
layer error control and packet sequence control.

2.1.4 Transport Layer

The transport layer accepts messages of arbitrary length from the session ayer,
segments them into packets, submits them to the network layer for transmission,
and finally reassembles the packets at the destination. The transport layer records
a sequence number in each packet, and uses the sequence numbers for detecting
lost packets and for ensuring that messages are reconstructed in the correct
sequence.

Transport layer guarantees transmission of data from one end to the othef. It decides
if the data transmission should take place on parallel paths or single path. The
overall function of transport layer is to provide transport service of the quality required
by the session layer entities, when quality is specified in terms of residual error rate.,
throughput, delay and other quality measuring parametres. The relationship of the
transport layer to the network layer and session layer is shown in fig. 2.5.
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 From Session Layer To Session Layer
Layer 5 data Layer 5 data

Transport Tranßport
Layr Laygr

H H H. HA H,

Layer 4 data Layer 4 data

Te Network layer From Network layer

FIGURE 2.5

Function of Transport layer are:

Segmenting (or packetizing) and reassembly
The transport layer creates packets out of the messages recevied from the
application layer./Packetizing is a process of dividing a long message into
smaller ones. A header is also added to each packet. After packetizing it
(transpot layer) submits these packets to network layer for transmission and
finally reassembles the packets at the distination.
Sequence Numbering
The transport layer records a sequence number in each packet and uses the
sequence number for detecting lost packets and for ensuring that messages
are reconstructed in the correct sequence.
B. Connection Control

Transport layer protocols are divided into two categories :

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 (a) Connection orlented delivery:Aconnection oriented transport layer
protocol establishes a connection i.e. virtual path between sender and
receiver.
(b) Connectionless delivery: Aconnectionless trancport protocol will treat
each packet independently. There is no connection between them. Each
packet can take its own different route.
Flow Control

We know that data link layer can provide the flow control.
Similarly transport
layer also can provide flow control, But this flow control is performed end to
end rather than across a single link.
5: Error Control

The transport layer can provide error control as well. But error control at
transport layer is performed end to end rather than across a single link./Error
correction is generally achieved through retransmission.
6. Muitiplexing if Needed
For optimum use of network connection, the transport entity may assign several
end-to-end transport connection to the network connection. This is known as
multiplexing.

2.1.5 Session Layer

7
This layer provides establishing, maintaining and teminating a.session or dialogue
between twO end systems. It creates the session, manages the packets sent back and
forth during the session and teminates the session. The dialogue type/session type is
also specifed by it i.e. half duplex or ful duplex The session layer also provides for data
synchronization and check pointing so that in the event of the network failure, only the
data sent after the point of failure need be resent. This layer also controls the dialogu
between two processes determining who can transmit and who can receive at what
point during the communication The relationship of the session layer to the transport
layer and the presentation layer is shown in figure 2.6.
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 From presentation Layer To presentation Layer
Layer 6 data Layer 6 data

Session Session
Layef Laye

H5
H5

syn syn syn syn syn syn

Layer 5 data Layer 5 data

To Transport layer From Transport layer

FIGURE 2.6
Functions of session layer are :
1. Session Establishment

The main function of session layer is to provide and estatblish connection

between session users, known as session. Asession between two computer
can be established for communication, file transfer, for remote login or to
Some other purpose.
Normal Data Transfer

It is the most important function of session layer. The exchange of data between
user entities mnay either be two way alternate (half duplex) or two way
simultaneous (full duplex).
3 Dialog Managemernt
In most of the cases the session connections are full duplex but the upper
layers some times communicate in half duplex modes too. In such cases t
session layer has to keep track of whose turn it is to talk and is known s
dialog management.
4 Session Release

For releasing the session connection one of the following four variations av
used:
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 User abort

Provider abort
Orderly release
Negotiated Release
5. Synchronization
The session layer allows a process to add check point i.e.
synchronisation
points into a stream of Jata. In case of crash during the
transmission of the
data can be retransmitted from the check point inspite of
the start.
retransmitting it trom

2.1.6 Presentation Layer

This layer converts data received from application layer into machine's native
internal
numeric fomatandencoded transmitted data into displayable forn for output. It
perforns
code conversions, text compressions, security encryption etc. on the
message. It
represents data according to the software/hardware environment of the node. For
example, presenting Unix formatted data in windows. The relationship between the
presentation layer and the application and session layer is shown in fig. 2.7.
From Application layer To Application layer
Layer 7 data Layer 7 data

9ssuuunenas
Prejentation Prejentation
Layr Layèr

Encoded, encrypted and H6 Decoded, decrypted and

compressed data decompressed data H6

Layer data Layer data

To Session layer To Session layer

FIGURE 2.7

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 Functions of Presentation layer are :
Data Compression
It reduces the number of bits to be transmitted by Compressing the dot
2. Data Conversion

It formats the data on different nodes as per their

environnment. Software/Hardware
3. Code Conversion

The fom and syntax (language) of the two communicating systems can be
different. For example one system is using the ASCIl code for file transter an
the other one uses IBM's EBCDIC. Under such conditions the presentation
layer provides the "translation" from ASCIlto EBCDIC and vice versa.
4. Data Encryption
It encodes data ina specific format soas it can not be understood by each
and every user or application..
Allthe receiver end, it performs the decompression, decoding and decryption.

2.1.7 Application Layer

This is the topmost layer of OSI architecture. It is the layer seen by the application
program and hence is user oriented layer which provides services to support end
user of network directly. A message to be sent across.network enters the OSl
model at this point and then travels down ward into physical layer then through
transmission line unto physical layer of destination &then upwards up to application
layer of receiver end system. It provides services that directly support user
applications such as database access, email and file transfer.
Some popular application layer protocols are X.400 (Electronic Mail Protocol), X.500
(Directory Server Protocol), FTP (File Transfer Protocol), and rlogin (Remote Log
Protocol). Fig. 2.8 shows the relationship of the application layer to the user and te
presentation layer.
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 User
User

Application
Layer X.500 Application
FTP X.400 Layer X.500 FTP X.400

Layer 7 data
Layer 7 data
To Presentation layer
From Presentation layer

FIGURE 2.8
Functions of Application layer are:
1.User Interface
It defines user interface to low
level layers and various application
processes.
2 Security
It is responsible for implementation of security checks at user entry points.
3 File Transfer

The application layer provides file transfer access and

management (FTAM)
which allows a user to %ASs files in a remote computer to
retrieve files from
a remote computei änd tO manage or control files in a remote
computer.
E-mail
It provides a basis for e-mail forwarding and string.
5. Database Access
It provides distributed database sources and access for global information
about various objects and services.

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2.2 Example of Data Transmission In 0SI Model
Let us assume that a process at computer A(sending node) wants to
send
Mto a process at computer B (receiving node).
1. The sending node's process builds the message Mand
am
passes
es ag
it
A to
application layer on its machine. The application layer software addsa he
(H,) to Mand passes theresulting
in fig. 2.9.
message to the presentation
layer as shown aheader
Computer
A Computer

L7 Data H7 L7 Data H7

L7 Data H6 L7 Data H6

4
L6 Data H5 L6 Data H5

L5 Data H4 L5 Data H4

L4 Data H3 4L4 Data H3

T2 T2 4
L3 Data H2 L3 Data H2

01010101010101010000010000 01010101010101010000010000

Actual Data Actual Received

Transmission Data
Transmission Medium

FIGURE 2.9

2. The information added by each layer is in the fom of headers or trailers. Heace
are added to the message at the layers 6, 5, 4,3, and2. Atraileris added at layer

3. At layer 1the entire package is converted to a form that can be transferreo

the receiving machine. At the receiving machine, the message is unwrapP
layer by layer with each process receiving and removing the data meant to
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 2.3 Advantages and Disadvantages of CSI Model
Advantages of OSI Model are
1. The OSI models Clearly defines
services. interfaces and protocols. It defines
that which services are
provided by which layer, how the processes
above a particular layer willaccess its situated
its protocols to do its job. functionalityand how a layer would use
The protocols of ISO model are better
hidden and can be replaced with other
suitable protocols easily.
3 It is truly a general model.

4.tsupports connection oriented as well as

connectionless services.
Disadvantages of OSI model are
Sessions and presentation layers are not of much use.
2. The OSl model was built first and then
protocols were designed to fit the need
of network architecture. So in real life
there isaproblenm of fitting protocol into
the model.

2.4 Examples of Networks

2.4.1 Internet
The term Internet has been coined from two terms,
Interconnectionand Network. A
network is simply a group of computers that are connected together for
sharing
information and resources several such networks have been joined together across
the world to form what is called as the Internet.

Ihus, Internet is a network of networks that are:

() Interconnected physically.
(W) Capable of communicating and sharing data with each other.
() Able to act together as a single network as shown in fig. 2.10.

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